1. Field of the Invention
This invention relates to a test pattern device useful in testing a radiation detector such as a scintillation camera, and a method of manufacturing the same.
2. Description of the Prior Art
Test pattern devices are used to calibrate and evaluate the performance of radiation detectors. Radiation detectors, especially nuclear radiation detectors like scintillation cameras, are widely used as medical diagnostic tools for detecting the radioactivity of an object under investigation, such as for determining the distribution of a radioactive isotope absorbed by a human body organ. Examples of scintillation camera systems to which the present invention finds application are the basic Anger-type scintillation camera (named for its inventor) described in U.S. Pat. No. 3,011,057, and improvements thereof. Radiation detectors are also used in radiation transmission applications, such as in X-ray and computed tomography diagnoses.
Test pattern devices which are known as "resolution bar patterns" serve as part of the quality control apparatus used for scintillation cameras to make quality assurance checks to monitor camera performance and detect malfunctions. These test devices comprise radiation opaque material configured in the form of a calibrated bar pattern. The test pattern device is positioned between a radiation source (such as a gamma radiation source) and the scintillation cyrstal of the camera. The test pattern device is thus exposed to radiation and the resulting scintiphotos are evaluated. Intrinsic "flood and resolution" checks (without collimator) are frequently made to verify the uniformity, linearity and intrinsic resolution of the camera. Likewise, collimated "flood and resolution" checks (with collimator) are also frequently made to uncover collimator damage (e.g. collimator septa damage) and verify collimator/camera system performance. For uncollimated checks, a mask or shield ring is used to outline the useful field of view and thereby to minimize the edge-packing artifact inherent during uncollimated operation. Such quality control checks of radiation cameras using test pattern devices are described, for example, in the operating manuals of the Siemens-Gammasonics, Inc. (2000 Nuclear Drive, Des Plaines, Ill., formerly called "Searle Radiographics, Inc.") scintillation camera models 6480 and 6478, sold under the trademarks "Pho/Gamma LEM" and "Pho/Gamma LFOV", respectively (Publication Nos. 710-000880/Rev. C and 710-000650/Rev.C).
The most frequently used material for the radiation opaque parts of prior art test pattern devices is a dense solid metal such as lead, although other materials such as tungsten powder have also been used. One prior art device is formed by machining slots to form a bar pattern in a solid lead body member. Another known device comprises lead bars arranged to form a calibrated bar pattern configuration within a closed radiation transparent body member. A typical configuration of this latter type device is the Searle Radiographics "Resolution Pattern #180-823108" which has a plurality of sets of parallel evenly-spaced bars of uniform width positioned within the plane of a plastic disc-shaped body member. The different sets of bars have different uniform spacings and widths and adjacent sets of bars are oriented perpendicularly to one another. Other configurations include patterns with continuously varying spacing, crosshatch patterns, and overlapping linear patterns which can be rotated with respect to each other to provide different Moire effect beat frequencies.
A typical prior art test pattern device is manufactured by forming grooves in a plastic or glass radiation transparent base member, the grooves being arranged in the desired bar pattern configuration. Machined or extruded bars of lead are then fitted into the grooves. The whole is then sealed by means of a cover plate secured to the grooved base member. The sealing provides a closed structure that is convenient for consumer use and is usable in both the horizontal and vertical positions.
In order to accurately test scintillation camera performance with such a test pattern device, the lead bars must not only be precisely machined or extruded to assure their correct width, but the spacing between the bars must be accurate. Prior art methods for making test pattern devices present difficult tolerance problems. Accurate bar spacing is affected by the tolerance of the bars and the clearance required for the bar to be inserted into the slots. Such tolerance difficulties become significant when typical bar phantom spacings and widths are on the order of 2-4 millimeters.
Prior art test pattern devices also take the form of calibration masks, such as those used for developing stored data for the correction of spatial nonlinearities inherent in converting the scintillations of Anger-type cameras into position coordinate electrical signals. Example of such calibration masks are described in U.S. Pat. Nos. 3,745,345 and 4,212,061 and comprise lead radiation opaque plates having calibrated apertures or gaps. Such calibration mask test pattern devices likewise suffer from the difficulties encountered in prior art manufacturing techniques where lead is used as the radiation opaque material.